1. Field of the Invention
The present invention relates to a magnetoresistive sensor device for detecting a change in a magnetic field induced by a moving body. More specifically, the present invention relates to a magnetoresistive sensor device which can be used as an on-vehicle sensor.
2. Description of the Related Art
As a magnetoresistive sensor device for detecting a change in a magnetic field induced by a moving body, there is a magnetoresistive sensor device which has magnetoresistive sensor elements formed of a giant magnetoresistive thin film formed on a signal processing circuit (integrated circuit: IC). As the giant magnetoresistive thin film in the magnetoresistive sensor device, an artificial lattice film comprising magnetic layers mainly containing Ni, Fe and Co alternately laminated with non-magnetic layers mainly containing Cu is used. In general, each of the magnetic layers and each of the non-magnetic layers of the artificial lattice film have a thickness of 10 to 25 Å, and the artificial lattice film is formed of 10 to 40 laminates, each including a magnetic layer and a non-magnetic layer. To match the lattice constant of the artificial lattice film with that of a surface of a base layer on which the artificial lattice film is to be formed, a buffer layer having a thickness of 10 to 80 Å is formed as the base layer for the artificial lattice film.
While the artificial lattice film having the laminate structure can achieve an extremely large magnetoresistance change rate of 20 to 30%, interaction between the very thin magnetic layer and the very thin non-magnetic layer is needed. Therefore, matching between the artificial lattice film and the base layer on which the artificial lattice film is to be formed becomes important. For example, an artificial lattice film formed directly on an IC having an uneven surface has a much lower magnetoresistance change rate than an artificial lattice film formed on a thermally oxidized film of a flat silicon wafer. When an artificial lattice film is formed on a flat oxidized film such as phosphorus silicate glass (PSG), if the PSG is left in the air for a long period of time before formation of the artificial lattice film, the surface state of the PSG is changed by the influence of moisture absorption or natural oxidation, which occurs on the surface of the PSG and the artificial lattice film formed on the surface of the PSG cannot achieve a satisfactory magnetoresistance change rate.
To cope with this, there is a magnetoresistive sensor device having a cured film of a special silicone polymer between an artificial lattice film and an IC (refer to JP 3626469 B, for example). This special silicone polymer can flatten the uneven surface pattern of the IC and match well with the artificial lattice film. Accordingly, the artificial lattice film formed on the cured film of the silicone polymer can achieve an excellent magnetoresistance change rate. Further, the magnetoresistive sensor device can achieve such high reliability that the magnetoresistive sensor can be used in an on-vehicle sensor by using an appropriate silicone polymer. To use the magnetoresistive sensor in an on-vehicle sensor, the magnetoresistive sensor device must retain an excellent magnetoresistance change rate even in a very severe environment such as a heat cycle endurance test in which the temperature amplitude is −40 to 140° C.
In JP 3626469 B the uneven pattern of the IC surface is flattened by applying a silicone polymer onto the uneven pattern of the surface of the IC. However, a silicone polymer having high reflowability (melt fluidity) must be selected to flatten the pattern completely. Therefore, the average molecular weight of the silicone polymer must be about 50,000 or less to ensure the reflowability of the silicone polymer.
When the ratio of an inorganic component in the silicone polymer is high, the brittleness of the cured film of the silicone polymer is increased. As a result, cracks are readily formed by stress in the cured film itself. The cured film must be made thin to suppress the formation of cracks, so the thickness of the cured film may be unsatisfactory to flatten the uneven surface pattern of the IC. Therefore, the content ratio of the inorganic component in the silicone polymer must be about 0.4 or less, that is, the content ratio of an organic component in the silicone polymer must be about 0.4 or more to ensure crack resistance.
Therefore, it has taken a great amount of time and labor to design a molecule of a silicone polymer which is excellent in both reflowability and crack resistance and which will match the magnetoresistive sensor elements well. As a result, the cost of a designed silicone polymer rises and a magnetoresistive sensor device cannot be manufactured at low cost.
When a silicone polymer which is excellent in both reflowability and crack resistance and which will match the magnetoresistive sensor elements well is not obtained, before the silicone polymer is applied to the uneven surface pattern of an IC, another material must be used to flatten the uneven surface pattern of the IC. That is, two or more different films must be interposed between the magnetoresistive sensor elements and the IC. In this case, as the silicone polymer does not need to have the reflowability, it is assumed that the degree of molecular design freedom increases and the cost of the silicone polymer slightly drops.
However, after the uneven surface pattern of the IC is flattened by a flattening film, the step of forming a cured film of the silicone polymer on the flattened IC is required. Therefore, the number of steps increases and long-time heat treatment is also needed for the reflow curing of the flattening film and the thermal curing of the silicone polymer. As a result, productivity thereof becomes low and the production cost thereof becomes high.
The cured film of the silicone polymer is a resin film. The surface of the cured film is oxidized and becomes inorganic when the cured film is exposed to oxygen plasma. As a result, the cured film becomes brittle and cracks are readily formed on the surface of the cured film. In a magnetoresistive sensor device having magnetoresistive sensor elements formed on the cured film of the oxidized silicone polymer, a satisfactory magnetoresistance change rate is not obtained. Therefore, in the step of etching the cured film of the silicone polymer (step of forming contact holes), it is difficult to use a photoresist film. This is because the step of ashing using oxygen plasma is generally required to remove the photoresist mask. When a metal mask is used, as the metal can be removed by wet etching, the oxidation of the surface of the cured film of the silicone polymer can be avoided but the number of production steps increases, thereby reducing productivity and boosting the production cost.